Cell body 



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Receptor 



Effector 



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are disturbed, muscular cells contract. As we watch the 

 hydra in the water, we can see the stimulation lead to 

 movements of the tentacles and of the whole body, al- 

 though we can see neither the nerve cells nor the 

 motor cells. 



Nerve Cells In man, as in all vertebrates, and in 

 other species of complex animals, the nerve cells, or 

 neurons, are clearly distinct from other kinds of cells. 

 A neuron may be compared to a muscle cell as a unit 

 of muscle, or to a gland cell as a unit of gland. More- 

 over, there are several kinds of nerve cells (see illustra- 

 tion in margins) : (1) Neurons that receive impressions 

 from the outside (for example, through the skin, the 

 eye, etc.) we may call receptors, or receivers of stimuli. 

 Since they transmit impulses toward the brain or spinal 

 cord, these sensory neurons, or receptors, are also called 

 ^/-ferent, that is, bearing toward. (2) Neurons that 

 arouse muscles or glands into action are called effectors 

 — effect-producing. Since effectors bear impulses away 

 from the cord or brain, they are also called ^/-ferent 

 neurons. (3) Neurons that connect afferent and efferent 

 neurons are called associative neurons, or simply con- 

 nectors. 



The whitish strands commonly called nerves reach 

 to all parts of the body, and some of them are large 

 enough for us to see without a microscope. Nerves con- 

 sist of many fibers bound together by connective tissue 

 and associated with blood-vessels and lymphatics. The 

 cell bodies are grouped in the brain, in the core of the 



TYPES OF NEURONS 



A typical nerve cell has a nucleus and 

 an extension, or axon, that ends in fine 

 treelike branching, or dendrites. The 

 endings are in close contact with 

 dendrites of other cells, or with sensory structures, or with muscles or glands 



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Sense 

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Muscle 



